The thrombotic and hemorrhagic complications in patients with chronic myelopro-liferative disorders are frequently attributed to the marked elevations in platelet counts that often accompany them. This association has prompted the early prophylactic control of platelet counts whether or not they are producing symptoms. Few systematic and comprehensive prospective analyses of these patients and their platelet functions have bee performed and their conclusions have been conflicting. We plan to examine a large group of these patients, examine their platelet funtions in detail and compare them to a population with secondary thrombocytosis.